Precision dual-beam cathode-ray tube



June 16, 1959 G. F. BARNETT 2,891,183

PRECISION DUAL-BEAM CATHODE-RAY TUBE Filed Deo. 1'7, 1954 2 Sheets-She et 1 IN V EN TOR. 61/) E BARNETT June 16, 1959 G. F. BARNETT v2,891,183

' PRECISION DUAL-BEAM CATHODE-RAY TUBE Filed Dec. 17, 1954 2 Sheets-Sheet z I 3 n I 45 v u IN VEN TOR. 61/) F- 5AKNE 7' 7' Wag/M 1477'0RNEM United States Patent Ofiice 2,891,183 Patented June 16, 1959 PRECISION DUAL-BEAM CATHODE-RAY TUBE Guy F. Barnett, Roslyn, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Pennsyl- Vania Application December 17, 1954, Serial No. 475,892

7 Claims. (Cl. 313-69) This invention relates to electron beam transducing tubes, and more especially it relates to cathode-ray tubes having a plurality of closely spaced electron beams.

The invention is in the nature of an improvement on the kind of electron tube disclosed in application Serial No. 428,744, filed May 10, 1954.

A principal object of the invention is to provide a precision cathode-ray tube having a plurality of closely spaced beams of high current density and substantially entirely free from beam interaction, whereby the beams can be focused by the same focusing system and can be deflected by the same beam deflection system.

Another object is to improve the operating characteristics and the precision assembly of dual beam color television picture tubes and the like.

As pointed out in said application Serial No. 428,744, ordinary cathode-ray tubes, such as are used as black and White or monochrome reproducers, are not practical for color television since the scanning beam spot, for a given fineness of detail, must be focused to much smaller size. Furthermore, the beam current or intensity requirements for a color picture tube are much more stringent than for monochrome tubes. The said prior application Serial No. 428,744 discloses a novel combination and organization of elements whereby a satisfactory color picture tube can be produced with utmost precision. In that type of tube it has been found that dimensional variations of even slight order, which are permissible in black and white or monochrome tubes, are not permissible in color tubes. The problem is even more complex where the color tube is of the kind employing two closely spaced sharply focused beams which must maintain their spacing at all times without undesirable interaction. While tubes made according to the said copending application have been found entirely satisfactory for the production of brilliant and sharp color displays on the tricolor screen, I have found that this performance can be improved by a simplification in the electron gun assembly.

Accordingly, one of the principal features of this invention is to provide a novel dual beam gun construction for a cathode-ray tube wherein ultra precision ceramic elements are provided for gauging and spacing the dual control grid strips from the cathode emission surface. Those ceramic elements effectively insulate the grid strips from the associated metal parts or electrodes, and at the same time provide a rigid clamping member to receive the clamping thrust of a retainer ring or the like, whereby the grid strips are clamped against a mica insulator which separates them from a special dual beam convergence electrode.

In accordance with another feature of the invention, the ceramic precision members are specially cut and grooved to decrease the likelihood of electrical leakage between the cathode and grids as a result of undesirable conductive material evaporated from the heated cathode to the surrounding ceramic parts.

Another feature relates to the novel organization, arrangement, and relative location and interconnection of parts, which cooperate to provide an ultra precise dual beam cathode-ray tube of high current density and with closely spaced non-interacting electron beams.

In the drawing, which shows by way of example one preferred embodiment,

Fig. 1 is a schematic view of a dual beam tube embodying the invention and wherein the electron gun is shown schematically;

Fig. 2 is a greatly magnified cross-sectional view of an .actual gun that may be used in the tube of Fig. 1;

Fig. 3 is a top plan view of one of the improved ceramic spacing and gauging insulator elements according to the invention;

Fig. 4 is a sectional view of Fig. 3, taken along the line 4-4 thereof;

Fig. 5 is a sectional view similar to that of Fig. 2 but taken at right angles to the sectional view of Fig. 2;

Fig. 6 is a sectional view of Fig. 5, taken along the line 6-6 thereof;

Fig. 7 is a magnified sectional view of part of the tricolor screen of the tube of Fig. 1.

While the invention will be described as embodied in a cathode-ray tube for the reproduction of colored pictures or images employing a phosphor screen of the tricolor strip kind, it will be understood that the invention can be used in other kinds of electron beam transducing tubes, such as storage tubes, multiplex signaling tubes, or any other tube where two discrete but closely spaced electron beams are to be acted upon by the same focusing system and by the same deflection system and without mutual interaction between the beams.

As indicated in Fig. 1, a tube according to the invention may comprise any well-known form of evacuated enclosing bulb consisting of the usual glass neck portion it) which is joined to the funnel-shaped portion 11, this funnel-shaped portion being closed off by the transparent viewing face portion 12. The opposite end of the neck portion It! has sealed thereto any well-known type of glass header 13 through which the various lead-in wires 14 are sealed preferably, although not necessarily, in circular array. Supported within the neck portion 10 in the manner to be described hereinbclow, is an electrode assembly according to the invention. This assembly comprises in general a dual-beam electron gun 15 which will be described in detail hereinbelow in connection with Figs. 26, and an elongated tubular anode 16. The anode 16 is of much longer axial length than that customarily employed in black and white cathode-ray tubes, and is supported firmly but resiliently against the inner wall of neck 11 at a plurality of spaced planes by means of the metal spring fingers to be described.

Gun 15 is, according to the invention, designed to develop a dual-beam, the two beams being indicated by the numerals 17, 18 (Fig. 1). Beam 17 will be referred to herein as the color transducing beam, while beam 18 will be referred to as the monitoring or indexing beam. Surrounding neck lltl is any well-known form of electromagnetic focusing yoke schematically indicated at 19 for focusing the dual-beams 1'7, 18, as respective minute spaced spots on the fluorescent screen Zil. Screen 20 may be attached to, or deposited directly on the interior surface of face plate 12, or if desired, it can be applied to any suitable light transparent sheet: which can be mounted within the tube, in close proximity to face plate 12.

Screen 265 may comprise a series of regularly repeated sets of tri-color strips 21, 22, 23, (Fig. 7) which extend parallel to one another. Preferably these strips are spaced slightly from each other, but need not necessarily be. Each of these tri-color sets comprises a strip 21 which, for example, may be of any well-known fluorescent material which fluoresces blue when struck by Patent No. 2,742,531.

the transducing beam 17; strip 22 may be of a material which fluoresces red, and strip 23 may be of a material which fluoresces green. In accordance with well-known color principles, even when the strips of each tri-color set are energized in time succession by the beam 17, the combination .of colors because of visual retention and inability to resolve lines at normal viewing distance and because of the phosphorescent effect, produce a resultant single visual color whose hue will be dependent upon the relative intensities with which the inividual color strips in each set are excited by the beam 17 The elemental width of the beam 17 in the direction of the horizontal line scan should not be greater than the Width of each color strip, and preferably is substantially less than in the conventional black and white picture tube, wherein the scanning beam, for substantially the same picture definition, can have a width approximately three times as Wide as the beam 17. Therefore, in the type oftubeaccording to this invention, it is necessary, for image brightness equivalent to the black and White tube, that the beam 17 have a peak intensity which is many times that required in such black and white tubes. Furthermore, ithasbeen found necessary to prevent inter-modulation between the dual-beams 1'7, 18. This inter-modulation prevention is achieved by a novel beam-convergence and beam-shielding electrode.

By any Well-known magnetic horizontal and vertical deflection yokes represented schematically at 24, surrounding the neck 10, the two beams 17 and 18 are moved, synchronously or in unison so as to scan the screen 20 in successive parallel lines extending substantially transverse to the length of the tri-color strips of screen 20.

All the fluorescent strips 21, 22, 23, etc., are provided with a thin electron permeable coating 20a, of low secondary electron-emissivity but which is electrically conducting and transparent to the beam 17. Typical of such materials is aluminum, magnesium, or beryllium. The coating 20a can be deposited so that it also acts as a mirror to reflect the fluorescent light from the fluorescent strips outwardly through the face plate 12 which face plate, of course, should be substantially uniformly transparent to all the desired visible light rays from the screen 20. Suitably deposited on the coating 20a on the side facing the gun 15 are spaced strips 2% of a material having a high secondary emissive power or at least of a power which is substantially higher than that of the coating 20a. For example, the strips 20b may be of gold or other high atomic number metal such as plati- .num or tungsten, or of an oxide such as magnesium oxide or any other equivalent high ratio secondary emitter. The secondary emission strips 20b are applied over the coating 20a but only in registry with fluorescent strips of a particular color, for example the secondary emission strips 20b may be applied so as to be in registry over the red fluorescent strips 23. By means of the coating 20a, the screen as a Whole can be connected to the positive pole of a suitable direct current power supply by means of'a conductor sealed through the wall of the cathode-ray tube bulb. For a detailed description of the action of the indexing beam 18 with respect to the secondary emission strips 20b, reference may be had to Serial No. 242,264, filed August 17, 1951, now Suffice it to say for the present that each time the indexing beam 13 strikes one of the secondary emission strips 20!), it produces a pulse of secondary electron current. This pulse can be picked up by a suitable electron collector electrode, such for example as a conductive coating on the interior Wall of the bulb adjacent to, but spaced from, the screen 20.

Because of the extremely small width of the scanning beam and because of the high beam intensity required, conventional gun constructions used in black and white tubes are not feasible for this type of tube. Furthermore, the parts of the gun must be made with extreme precision and yet capable of assembly in such a way that in the final assembled unit the desired uniformity of precision is maintained, even between large batches of such guns.

For that purpose, the gun construction such as illustrated in the drawing has been devised and has been found to produce the desired results. Referring to the drawing, the gun includes a cathode sub-assembly 25 consisting of a tubular metal sleeve 26 of nickel or the like, which is closed off at its forward end by metal cap 27 also of nickel which may be welded to the sleeve 26.

a The cap 27 may have a fiat surface or it may have a rounded or dome-shaped contour. If desired, the elements 26 and 27 may be formed from a single extruded metal piece. The cap 27 is of a sufficient thickness so that it maintains its close spacing with respect to the adi jacent control grids even when it is raised to electronemitting temperature by the usual heater element (not shown) mounted within sleeve 25. Cathode sleeve 26 passes through a corresponding central opening in a disc 28 of any non-conductive refractory ceramic such for example as Lavite, Steatite, etc., and is held firmly in place in the ceramic for example by peripheral beads 28a, 29, formed in the body of the sleeve 26 which beads abut against the respective opposite faces of disc 28.

One of the important items of precision which enables the gun to produce the desired results is that the front surface of ceramic 28 must be absolutely free from any undesired surface non-uniformities. For example, in the case of the flat surface of ceramic 28 the surface flatness should be to an accuracy of .0001 inch because this face of the ceramic forms the basic reference plane upon which the precision assembly of the remaining parts depends. For that reason, ordinary ceramic surfaces should be avoided. For example, there may be high and loW points and if these surface irregularities occur it may not be possible to maintain the desired precision of location and spacing of the various parts of the gun. In order to avoid this difliculty the ceramic 28 is preferably surface ground by any technique Wellknown in the precision lapping machine art. Forexample, the ceramic may be lap ground in a machine used in lapping the surfaces of piezo crystals and the like so that the flatness of the front face of ceramic 28 is within an accuracy of .0001 inch. Therefore, when the cathode sleeve 26 is assembled in the ceramic, the plane of the flat face of cap 27 is parallel to the front face of the ceramic within plus or minus .0002 inch. It will be understood, of course, that the invention is not limited to a cathode having a flat face adjacent the control grids. If desired, the face or cap on the cathode may be dome-shaped so as further to reduce any possibility of substantial change in spacing between the cathode and grids as the cathode is heated.

The external face of cap 27 is then coated by any well-known process used in the coating of cathodes for cathode-ray tubes, for example by spraying a mixture of barium and strontium compounds which are eventually broken down in the conventional processing of cathoderay tubes to produce electron emissive oxides.

Here again it has been found necessary to subject the cathode coating to a precise shaving operation. Therefore, when the coated cathode is assembled and anchored in ceramic 28, the distance from the flat forward gauging face of that ceramic to the surface of coating 34 is held to a fixed value plus or minus .0001 inch.

The above-described precision cathode sub-assembly 25 is arranged to be concentrically supported within a cylindrical metal member 35 having a cylindrical wall 36 which is a close fit around the ceramic 28.. The forward end of member 35 is bent at right angles to former flat annular flange 37 forming a central window.38. Wall 36 adjacent the forward end of member 35. is provided with diametrically opposite cutouts or windows 39, 40 through which are arranged to be radially inserted a pair of flat metal control grid strips 41, 42. Each of these strips has a right-angled bent-back portion 43, 44, to which respective flexible metal connector strips 45, 46, can be welded, these flexible strips in turn being welded at their opposite ends to respective control grid leadin prongs 47, 48, sealed vacuum-tight through the glass header 13.

As shown in the greatly enlarged sectional view of Fig. 6, the control grid strip 41 has a grid, aperture149" through which beam 17 passes for intensity control. Likewise, grid strip 42 has a grid aperture 50' through which the beam 18 passes. Each grid aperture is located close to the edge. of its strip so that the distance between the center lines of the two. grid openings is approximately .026 inch. Likewise, when the grid strips are finally assembled in place, their adjacent edges are spaced apart by a minute gap 53, for example of from .0015 to .005 inch. It has been found that this small gap enables the two grids to be insulatingly spaced from each other, and yet it is of suflicient narrowness to provide an effective barrier against the passage of electrons therethrough.

However, because of the minuteness of gap 53, it has been found necessary to design the insulated parts which contact the grid and the cathode so that the possibility of conductive current leakage between the grid and cathode is substantially eliminated. Furthermore, it is necessary: to maintain the strips 41, 42, free from mechanical distortion and with a minimum spacing between the grids and cathode. In order to insulate and at the same time to space the control grid strips 41, 42, from the next adjacent dual-beam convergence and inter-beam. shielding electrode 55, there is provided a .flat mica insulator ring 56 having a thickness of 005-010 inch. Ring 56 has a central enlarged circular opening 57 provided with diametrically opposite slots 58, 59, and is assembled in place so that slots 58, 59, overlie the gap 53, the said slots being substantially wider than the gap 53, thus reducing the likelihood of leakage across that gap. From the foregoing it will be seen that the spacing between the composite electrode and the flat control grids is only a small fraction of the distance between the center lines of the respective grid openings. Because of this substantial difference in relative spacings it is possible for the electrode 55 to act as an electrostatic shield whereby variations in potential of one grid do not cause any modulation of the beam passing through the other grid.

The composite beam convergence and inter-beam shielding electrode 55 is arranged to seat against the inside face of. flange 37, and therefore the outside diameter of electrode 55 closely fits the inside diameter of cylinder 36. As shown more clearly in the sectional view of Figs. 2. and 5, electrode 55 has a thickness which is many times greater than the thickness of the grid strips and of the insulator spacers and also is many times thicker than the thickness of face 37 of member 35. Thus, electrode 55, in addition to performing the various electric functions to be described, also acts as a solid backing or stiffener for the entire gun. Preferably, the forward marginal edge 60 is roundly chamfered to fit snugly within the corresponding curved inner edge 61 of member 35.

The central part of the forward face of electrode 55 is formed with a circular recess 62 having a depth of approximately .02 inch. The unrecessed thickness of electrode 55 is provided with two spaced openings 63, 64, symmetrically located on opposite sides of the center line of the electrode, thme twoopenings being spaced by the unperforated central portion 65. Openings 63, 64, are spaced apart on centers a distance which is the same asthespacing between the centers of control grid openings49, 50. Similarly, the openings 63,64, each, has

a cylindrical portion 66 and a forward frusto-conical portion 67. The two'beams 17, 18, which emerge through the respective grid openings 49 and 50, would ordinarily diverge away from the central longitudinal axis of the gun. However, by choosing a suitable diameter and depth. for the recess 62, and by choosing a suitable thickness for the insulator spacer 56, and by applying the proper potential to electrode 55, that electrode acts as a convergence electrode for the beams. This causes the beams after they leave their respective control grid apertures to be subjected to a converging action. In the absence of such a convergence electrode, the beams after leaving their respective and closely spaced grid apertures would diverge. These unexpected and novel results are obtained by locating the said composite shielding and convergence electrode 55 in a region extremely close to the control grids and by applying to that composite electrode a potential which is substantially the same. as the inherent field position potential at that region. In other words, the composite electrode 55 acts as an electrostatic shield between the two dual grids while at the same time it acts to converge the emergent beams, and without draining any beam current, and without exerting any focusing or defocusing action on the beams. Thus, the two beams maintain their accurate spaced trajectories so that they can be focused by a common focusing yoke 19 on to the fluorescent screen, with the same spacing regardless of the portion of the fluorescent screen 20 scanned by the beams. Furthermore, while electrode 55 acts as a single or common convergence electrode to maintain the desired spacing between the two focused beams, it also acts through its central unperforated portion 65 as an electrostatic shield to prevent cross-modulation between the two beams. It has been found that in the absence of this central portion 65, the two beams may undesirably cross-modulate each other when used in certain types of circuits. To achieve this shielding action, it is desirable that the convergence electrode 55be extremely close to the grid openings49, 50, and this also minimizes the beam current drawn by it. Also the electrode 55 is preferably maintained at a potential substantially the same as that of the region in the anode-cathode field in which it is located, so that the configuration of the equi-potential lines is not substantially altered by its presence, and it does not exert any substantial focusing or defocusing action on the beam. More particularly the face of electrode 55 confronting the cathode 27 is preferably kept at this potential.

In order to support the grids in their predetermined precise closely spaced planar relation with respect to the cathode, there is provided an annular ceramic ring 68 (see Figs. 3 and 4). Furthermore, its opposite: flat faces are ground, for example, by lap-grinding to perfect smoothness and flatness. This ring is specially shaped and cut to reduce the possibility of leakage between the cathode and the grids. One of the chief causes of such leakage is the sublimation or vaporization of conductive material from the cathode and adjacent metal parts, which material when deposited tends to destroy the desirable characteristics of the gun. In accordance with a feature of the invention, the ceramic ring 68 has its forward end radially extended to form an integral flange 68a, the flat face 68b of which is undercut with a pair of radially extending notches 69, 70, each of which has a tapered or inclined bottom Wall 71, 72. The rear flat face of ring 68 is annularly recessed to fonn an inclined shoulder 73. The inner peripheral face of ring 68 isalso provided on diametrically opposite sides thereof with a pair of non-radially extending slots 74, 75. The particular angle of these slots is not critical so long as the slots do not extend substantially radially with respect to the cathode sleeve 26. The various slots and notches are specially cut and arranged as shown so as to cast shadows where sublimation or vaporization of conductive material would be likely to deposit. In other words, these shadowed areas break the continuity of any conductive surface represented, by the stippled areas, which might otherwise tend to form on the interior of the ceramic ring 68 and on the surface of the cathode ceramic ring 28. If a completely unnotched insulator ring were used to space the cathode 26 from the grids 41, 42, cathode material, whether in the form of emission material or other conductive material vaporized from the cathode, would be ejected therefrom in all radial directions against the full height of the internal wall of the ring, and would also be ejected over the face of ceramic ring 28 as indicated atthe stippled areas in Figs. 2, 3, 5, and 6. The continued deposition of this material would eventually produce a continuous conductive leakage path between the cathode and the grids.

On the other hand, by forming the downwardly inclined shoulder 73 on the ring 68, there is always a shadowed area in the corner junction between the ring 68 and the ring 28, and this area is positively protected against deposition of conductive material. By providing the ceramic spacer ring with one or more non-radially directed notches such as notches '74, 75, it is not possible for the deposited material to form a continuous conductive surface since that surface is always shadowed against deposition by the non-radial notches.

It should be observed that if simple radial notches were provided on the interior face of member 68, as compared with the non-radial notches 74, 75, the desired discontinuity could not be prevented since the material would eventually deposit within the radial notch. Furthermore, the forward face of the cathode ceramic ring 28 is also provided with a relatively narrow annular recess 77, which lies under the shadowing notched shoulder 73. This further decreases the likelihood of formation of a continuous conductive deposit on ring 28.

By providing the ceramic ring 68 with an integral flange 68a that fits within the metal cylinder 36, this flange can 'be used as a thrust receiving member to hold the electrodes in their clamped stacked array, Furthermore, I have found that with this particular construction it is possible to assemble the flat grid strips 41, 42 in direct contact with the upper flat lap-ground face 68b, thus avoiding the use of a separate mica ring for that purpose. I have found that the use of such mica rings between such closely gauged electrodes, as between the cathode emission surface and the flat grids, is undesirable not only because of the tendency of the mica to flake and crack but also because the mica has a smooth surface which may permit lateral slippage between the grid strips when the parts are assembled and fastened in place. By using a lap-ground ceramic flanged member, such as disclosed, it is possible to have a direct metal-to-ceramic contact between the flat grid strips and the ceramic gauging surface, without the danger of grid slippage during assembly or operation of the tube.

Furthermore, by precision lap-grinding of the forward flat surface 68b of ring 68, the desired precise close spacing between the cathode coating and the flat grids 41, 42, is permanently maintained. It has been found practical with this arrangement, therefore, to provide a spacing between the cathode and grids of as little as .003 inch plus or minus .0004 inch, while preserving the necessary freedom from deposit leakage.

The manner of assembly of the various parts of the gun is as follows. The single composite beamconvergence, anti-cross-modulation electrode and stiflener element 55 is inserted into member 35 until it seals against flange 37 and welded in place. Disc 56 is then inserted. The two grid strips 41, 42, are then inserted from opposite radial directions through windows 39 and 40. Ceramic gauging ring 68 is then inserted into member 35 until its flat face 68b rests against the grid strips. Then, a steel retainer ring 78, having an annular bowed top 79 is inserted into member 35 and sprung against flange 68d." Ring 78 should'be made of very thin metal stock, for example of .008-inch thickness and-with its outside diameter a' close fit to the internal diameter of member 35, so that the ring 78 can be sprung into place and pressed tightly against the flange 68a. Thereupon the peripheral wall of ring 78 can be welded to the cylindrical wall 36 of member 35 at a plurality of spaced points around the periphery.

The cathode sub-assembly 25 is then inserted into member 35 until the cathode ceramic ring 28 abuts against the lower face of ceramic ring 68. Then the cathode sub-assembly retainer ring 80 of any suitable design is inserted. The retainer ring, therefore, provides sufficient frictional gripping to retain the various elements in their stacked position. 7

When the header 13, which carries the mount including the gun 15 and the anode 16, is sealed tothe lip of the neck portion 6, the bulb is subjected to the usual exhaust, bombarding and gettering processes well-known in the art, the exhaustion taking place through a suitable exhaust tubulation (not shown) forming part of 'the header 13. This tubulation is in the well-known manner sealed off when the processing has been completed. A conductive coating 112 extends internally along the neck 10 and part of the funnel-shaped portion of the bulb and is contacted by the fingers 108 to form a continuation of anode 16. For a detailed description of how such a tube can be used to reproduce a color subject matter on the screen 20 under control of received tri-color video signals applied to grid 41, reference may be had to application Serial No. 242,264, filed August 17, 1951.

From the foregoing, it will be seen that there is provided a cathode-ray tube having the desired minuteness of cross section in the transducing beam 17 and in the monitoring or indexing beam 18, While maintaining the required beam intensity for peak color excitation and also while preserving the fixed space relation between the two beam trajectories without cross beam intermodulation. It has been found that the desired conductive electrical isolation of the extremely close grids is obtained without danger of leakage, and it is possible to reduce the fluorescent spot by as much as five times that obtainable with ordinary cathode-ray guns. Furthermore, these advantages are obtained by using relatively simple electrode structures and assembly operations. It has also been found possible with the tube as disclosed to produce high definition colored television pictures with utmost brilliancy of hues.

While in the foregoing the grid strips 41 and 42 are arranged with the line joining the grid openings parallel to the length of the tri-color strips of screen 20, it will be understood that those grid openings may be oriented at any other angle with respect to the tri-color strips so long as the grid apertures are maintained in the same plane and are maintained the same distance with respect to the cathode emission surface.

As explained in said application Serial No. 242,264, filed August 17, 1951, the beam 18 acts as an indexing beam which, each time it strikes the secondary emission strips 20b, causes a pulse of secondary electrons to be emitted therefrom. This pulse then serves as an indication of the position of the transducing beam 17 with respect to the particular set of tri-color strips being scanned thereby; and this pulse can be used to overcome any nonhomogeneity or non-linearity in the horizontal deflection scanning field, or it can be used to overcome any nonuniformity in the width of the color strips. This indexing pulse can also be used as described in said applica tion Serial No. 242,264, filed August 17, 1951, to compare the time phase relation between the received tricolor video signals and the special phase between the transducing beam 17 and the respective tri-color strips which it is scanning. V The metal coating 20a for the screen 20 extends beyond the margin of that screen and is provided with a suitable external metal contact button 114 for connection to a suitable potential tap on a direct current power supply. For example, that tap may be approximately 25 kilovolts.

The anode coating 112 is likewise provided with an external contact button 120 which is connected to a suitable tap, for example of 3G kilovolts. The negative terminal of the D.C. source may be connected to the cathode sleeve 26 and grounded. The control grids 41 and 42 may be suitably biased with respect to the cathode 26, for example to potentials in a range from 75 to 200 volts. The composite shielding and convergence electrode 55 can be connected to a suitable potential tap, for example of 600 volts, corresponding, as hereinabove mentioned, to the potential of the region of the anode cathode held in which it is located. The screen 29 is coupled through a suitable coupling condenser to suit able indexing circuits from which indexing signals are derived and supplied to color signal control circuits to control the application of color signals from the tricolor signal source to grid 41 as described in the above-identified application Serial No. 242,264, filed August 17, 1951.

As described in said application Serial No. 242,264, grid 42 is supplied with a high frequency modulating signal from any suitable index beam control circuits.

By this arrangement it is possible therefore to keep the color transducing beam 17 and the monitoring beam 18 always at the proper spacing and with the desired minuteness of spot size on the screen 26 and accuracy of registration between the transducing beam 17 and the correct color strip is assured at all times.

Various changes. and modifications may be made in,

the disclosed embodiments without departing from the spirit and scope of the invention. For example, while reference is made herein to the ceramic disc 28 having a flat surface, it will be understood, of course, that the desired precision spacing may be obtained by using an unground ceramic disc which can be provided with three or more integral surface projections, the outer faces of which, however, are ground flat so as to provide the necessary accuracy of abutting contact. Furthermore, if desired, the ceramic disc 28 and the ceramic ring 68 may be made in one piece and the upper face, 68b of ring 68 can be ground to the desired flatness and accuracy.

What is claimed is:

p 1. A dual-beam electron gun comprising a cylindrical metalmember having at one end an inwardly extending annular flange defining an aperture for the emergence of twoclosely spaced cathode-ray beams, said metal member having a pair of diametrically opposite windows in its peripheral wall to receive a pair of control grid strips in substantially coplanar array, a ceramic insulator ring Within said metal member, said ring having one end precision groundand formed with a radially extending integral flange, said grid strips being in direct flat surface contact with said ground end of said ceramic ring, a first metal retainer ring within said metal member and engaging said flange to clamp said grids between said ground end of said ceramic ring and said annular flange of said metal member, a cathode sub-assembly comprising a ceramic block having a cylindrical cathode sleeve centrally anchored therein, said block being in direct abutting contact with the end of said ceramic body opposite said flange, the abutting surfaces of said block and said ring being precision ground to maintain a precise close spacing between the emitting end of said cathode sleeve and said grids, and a second metal retainer ring within said metal member and abutting against said ceramic block for maintaining said block and said ceramic ring in contact, said ceramic ring surrounding said cathode sleeve in spaced relation thereto, said ceramic ring being provided with a shadowing notch extending along the length of said cathode sleeve and a second notch surrounding said cathode sleeve to prevent the deposition pf conductive material from said sleeve in a continuous 10 path along the surface of said ceramic ring and said ceramic block.

2. A dual-beam electron gun comprising a cylindrical metal member having at one end an inwardly extending annular flange defining an aperture for the emergence of two closely spaced cathode-ray beams, said metal member having a pair of diametrically opposite windows in its peripheral wall to receive a pair of control grid strips in substantially coplanar array, a ceramic insulator ring within said metal member, said ring having one end, precision ground and formed with a radially extending integral flange, said grid strips being in direct fiat surface contact with said ground end of said ceramic ring, a first metal retainer ring within said metal member and engaging said flange to clamp said grids between said ground end of said ceramic ring and said annular flange of said.

metal member, a cathode sub-assembly comprising. a ceramic block having a cylindrical cathode sleeve centrally anchored therein, said block being in direct abutting contact with the end of said ceramic body opposite said flange the abutting surfaces of said block and said ring being precision ground to maintain a precise close spacing between the emitting end of said cathode sleeve and said grids, and a second metal retainer ring within said metal member and abutting against said ceramic block for maintaining said block and said ceramic ring in contact, said ceramic ring surrounding said cathode sleeve in spaced relation thereto, sa'id ceramic ring being provided with a shadowing notch extending along the length of said cathode sleeve and an undercut annular notch in the end of said ceramic ring which abuts against said ceramic block to prevent the deposition of conductive material from said sleeve in a continuous path along the surface of said ceramic ring and said ceramic block.

3. A dual-beam electron gun according to claim 2 in which said ceramic body is formed with an annular notch in a region shadowed from said cathode by said ceramic ring.

4. A dual-beam electron gun comprising a cylindrical support member having at one end an inwardly extending annular flange defining an aperture for the emergence of two closely spaced cathode-ray beams, a pair of control grid strips, said support member having a pair of diametrically opposite windows in its peripheral wall to receive said pair of control grid strips in substantially coplanar array, a ceramic insulator ring within said sup port member, said insulator ring including a body portion and an integrally formed flange extending radially from one end of said body portion, the. end of said insulator ring including said flange being precision formed to pro vide a precision flat surface, said grid strips being in direct fiat surface contact with said precision flat surface of said insulator ring, a first retainer ring within said support member, said first retainer ring having a first portion engaging said peripheral wall of said support member and a second inwardly extending portion engaging the surface of said insulator ring flange remote from said grid strips to clamp said grid strips between said precision formed end surface of said ceramic ring and said annular flange o f said support member, said inwardly extending portion of said retainer ring being formed with an aperture for receiving and positioning said body portion of said ceramic ring in coaxial alignment with said support member, a cathode sub-assembly comprising a ceramic block having a cylindrical cathode sleeve centrally anchored therein, said block being in direct abutting contact with the end of said ceramic body opposite said flange, the said abutting surfaces of said block and said insulator ring being precision formed to maintain a precise close spacing between the emitting end of the cathode sleeve and said grid strips, and a second retainer ring within said support member and abutting against said ceramic block for maintaining said block. in contact with said insulator ring.

5. A dual-beam electron gun comprising a cylindrical metal member havingat one end an inwardly ext'endin'g annular flange defining an aperture for thee'me'rgence of two closely spaced cathode-ray beams, said metal member having a pair of diametrically opposite windows in its peripheral wall to receive a pair of control grid strips in substantially coplanar array, a ceramic insulator ring within said metal member, said insulator ring including a body portion and an integrally formed flange extending radially from one end of said body portion, the end of said insulator ring including said flange being precision ground to provide a precision flat surface, said grid strips being in direct flat surface contact with said precision flat surface of said insulator ring, a first metal retainer ring within said metal member, said first retainer ring having a first portion engaging said peripheral wall of said metal member and a second inwardly extending portion engaging the surface of said insulator ring flange remote from said grid strips to clamp said grid strips between said ground end surface of said ceramic ring and saidrannular flange of said metal member, said inwardly extending portion of said retainer ring being formed with an aperture for receiving and positioning said body portion in coaxial alignment with said metal member, a cathode sub-assembly comprising a ceramic block having a cylindrical cathode sleeve centrally anchored therein, said block being in direct abutting contact with the end of said ceramic body opposite said flange, the said abutting surfaces of said block and said ring being precision ground to maintain a precise close spacing between the emitting end of the cathode sleeve and said grid strips, and a second metal retainer ring within said metal member and abutting against said ceramic block.

6. A dual-beam electron gun comprising a cylindrical support member having at one end an inwardly extending annular flange defining an aperture for the emergence of two closely spaced cathode-ray beams, a conductive double-beam convergence and inter-beam shielding electrode disposed within said support member and in contact with said annular flange, said electrode being formed with a pair of apertures therein for the passage of said beams, a pair of control grid strips, said support member having a pair of diametrically opposite windows in its peripheral wall to receive said pair of control grid strips, each of said grid strips having a single minute grid aperture formed therein in alignment with the respective one of said pair of apertures in said beam shielding electrode, means insulating said convergence electrode from said control grid'strips, a ceramic insulator ring within said support member, said insulator ring including a body portion and an integrally formed flange extending radially from one end of said body portion, the end of said insulator ring including said flange being precision formed to a flat surface, said grid strips being in direct flat surface contact with said precision flat surface of said insulator ring, a first retainer ring within said member, said first retainer ring having a first portion engaging said peripheral wall of said support member and a second inwardly extending portion engaging the surface of said insulator ring flange remote from said grid strips to clamp said grid strips, said convergence electrode and said insulator means between said precision formed end surface of said ceramic ring and said annular flange of said support member, said inwardly extending portion of said retainer ring being formed with an aperture for receiving and positioning said body portion of said ceramic ring in coaxial alignment with said support member, a cathode subassembly comprising a ceramic block having a cylindrical cathode sleeve centrally anchored therein, said block being in direct abutting contact with the end of said ceramic body opposite said flange, the said abutting surfaces of said block and said insulator ring being precision formed to maintain a precise close spacing between the emitting end of the cathode sleeve and said grid strips, and a second retainer ring within said support member and abutting against said ceramic block for maintaining said block in contact with said insulator ring.

7. A dual-beam electron gun comprising a cylindrical metal support member having at one end an inwardly extending annular flange defining an aperture for the emergence of two closely spaced cathode-ray beam, a conductive double-beam convergence and inter-beam shielding electrode disposed within said metal support member and in contact with said annular flange, said electrode being formed with a pair of apertures therein for the passage of said beams, a pair of control grid strips, said support member having a pair of diametrically opposite windows in its peripheral wall to receive said pair of control grid strips, each of said grids having a single minute grid aperture formed therein in alignment with the respective one of said pair of apertures in said beam shielding electrode, means insulating said convergence electrode from said control grid strips, a ceramic insulator ring within said support member, said insulator ring including a body portion and an integrally formed flange extending radially from one end of said body portion, the end of said insulator ring including said flange being precision ground to a flat surface, said grid strips being in direct flat surface contact with said precision flat surface of said insulator ring, a first retainer ring within said member, said first metal retainer ring having a first portion engaging said peripheral wall of said metal support member and a second inwardly extending portion engaging the surface of said insulator ring flange remote from said grid strips to clamp said grid strips, said convergence electrode and said insulator means between said precision ground end surface of said ceramic ring and said annularflange of said support member, said inwardly extending portion of said retainer ring being formed with an aperture for receiving and positioning said body portion of said ceramic ring in coaxial alignment with said metal support member, a cathode subassembly comprising a ceramic block having a cylindrical cathode sleeve centrally anchored therein, said block being in direct abutting contact with the end of said ceramic body opposite said flange, the said abutting surfaces of said block and said insulator ring being precision ground to maintain a precise close spacing between the emitting end of the cathode sleeve and said grid strips, and a second retainer ring within said support member and abutting against said ceramic block for maintaining said block in contact with said insulator ring.

References Cited in the file of this patent UNITED STATES PATENTS 2,540,080 Janis Feb. 6, 1951 2,570,165 Shekels Oct. 2, 1951 2,623,189 Diemer Dec. 23, 1952 2,712,087 Fite et al. .Tune 28, 1955 2,735,032 Bradley Feb. 14, 1956 2,784,334 Barnett Mar. 5, 1957 

